Signaling system



y 1933- E. K. SANDEMAN El AL 1,911,850

SIGNALING SYSTEM 2 Sheets-Sheet 1 Filed Dec. 28, 1929 5 K. SANDEMAN R.0. CARTER A UPNEY May '30, 1933- E. K. SANDEMAN ET AL 1,911,850

SIGNALING SYSTEM Filed Dec. 28. 1929 2 Sheets-Sheet 2 EKSANDEMAN //VlEN7'OPS. Ra CARTER A OP/VE Y Patented May 30, 1933 UNITEQ STATES PATENTOFFICE EDWARD KENNETH SANDEMAN, OF LONDON, AND ROBERT OWEN CARTER, OF

ALDWYCH, LONDON, ENGLAND, ASSIGNORS T0 WESTERN ELECTRIC COMPANY, IN-

CORPORATED, OF NEW YORK, N. Y., .A CORPORATION OF NEW YORK SIGNALINGSYSTEM Application filed December 28, 1929, Serial No. 417,203, and inGreat Britain January 9, 1929.

This invention relates to wave transmission systems and moreparticularly to transmission of the energy of given waves or impulses bymeans of waves of lower frequency than those whose energy is to betransmitted. In certain cases it is necessary or advantageous to reducethe width of the band of frequencies (such as the speech frequency band)before transmission in order to bring the frequency range to betransmitted within the efficient transmission limits of the transmittingmedium used. It is necessary in some cases to shift in the frequencyspectrum the frequency of the band to be transmitted. This may be donebefore or after frequency band reduction.

In numerous transmission systems it is de sirable to reduce the widthand/or the position in the frequency spectrum of the band of frequenciessent over the transmission medium; for instance if the transmissionmedium is a submarine cable (loaded or not) it is well known that theattenuation of such H cables increases so rapidly with frequency that itis practically impossible to transmit telephone conversation over longdistance submarine cables. Likewise in radio trans mission systems it isalso necessary to occupy as small a frequency band as possible for eachtransmission channel, in order to prevent interference between thevarious channels or stations in simultaneous use.

Several systems have been suggested for frequency band reduction. Forinstance, in British Patent No. 226,338 a signaling system is disclosedin which the band of frequencies of the signals sent over thetransmission medium is reduced at the sending station by means of aplurality of modulators and restored at the receiving station to theiroriginal range. Another arrangement having the same object is disclosedin British Patent No. 259,328.

The present invention has for its object the provision of an improvedwave transmission system of the type referred to above which willprovide a high grade of performance.

A feature of the present invention. consists in a signaling systemwherein the number of different frequencies, as well as the frequencyband width necessary for transmitting waves or signals such as spokenwords or the like, is reduced by periodically and with a suitable speedpicking up portions of the frequency spectrum, of the wave to betransmitted, by scanning means movable with time relatively to saidspectrum. The said scanning means may be if necessary provided withfrequency changing means such as modulators (with constant or varyingmodulating frequencies) for bringing the frequencies of the componentsof the waves picked up by the scanning means Within a smaller frequencyband width than that of the original waves to be transmitted; and thesaid reduced frequency band may then be shifted in position to occupy inthe frequency spectrum a range within the efiicient limits of thetransmitting medium. The said selected portions of the waves aretransmitted in turn with time selectivity over one channel, to thereceiving station where the transmitted portions of the waves arerestored to their original frequency range, and if necessary the saidportions are reproduced a number of times to compensate for the portionsof the frequency range of the waves which are not transmitted.

According to another feature of the invention the frequency band of thewaves to be transmitted (which will be for convenience termedhereinafter the speech band) is moved, for instance step-by-step orcyclically in front of a pick up device or scanning means; whereasaccording to a further feature of the invention it is the pick up devicewhich is moved relatively to the speech band.

A further feature of the present invention consists in a speech bandreduction system in which the energy spectrum of the signal or signalsto be transmitted is scanned periodically (continuously or not) by aband pass filter having fixed cut off frequencies. This is accomplishedas a result of the conversion of said signal or signals into amodulation product (e. g. a side band) of a periodically varying carrierfrequency or frequencies in such a manner that the essential frequenciesof the signal or signals to be transmitted during a period. of thevarying carrier frequency or frequencies are brought within the passrange of said filter.

Another feature of the invention resides in a system of the typereferred to in which the reduced frequency band is shifted in positionin the frequency spectrum, for instance by modulating said reducedfrequency band with one or several suitable modulating freq uencies soas to bring said band into the desired position in the frequencyspectrum.

According to a further feature of the inventi on the signals to betransmitted are impressed upon a plurality of modulators renderedoperative in turn and supplied with continuously (e. g. cyclically)varying car rier frequencies which vary according to such a law thatduring a cvcle of variation of the carrier frequencies at least portionsof the modulation products (e. g. a side band) fall within a range offrequencies of width smaller than the range of frequencies of thesignals to be transmitted, the signals or portions thereof thuscompressed in their frequency range being transmitted to the receivingstations where the signals are restored to their original frequencyrange.

According to another feature of the invention, the signals to betransmitted or portions thereof are impressed in turn and at a suit ablerate at the sending station upon a plurality of modulators, each ofwhich is sup plied with a constant frequency of such a value that thesignals or portions thereof in pressed upon said modulators are broughtwithin a frequency band width smaller than that of the original signals;and the said signals or portions thereof reduced in their frequency bandwidth are transmitted in turn to the receiving station where they are impressed synchronously upon a plurality of oscillators supplied withsuitable frequency for restoring the signals to their original frequencyrange.

According to a still further feature of the invention the speech bandmay be scanned by means of a filter of variable cut off frequencies.

According to a further feature of the invention, the synchronizingdevices at the sending and receiving stations may be dispensed with andreplaced by several transmission channels which are equal in number orless than the number of sub-bands in which the speech frequency band isdivided. For instance, if the speech frequency band is divided in saythree portions or sub-bands which are transmitted over three separatechannels, the said three separate channels may be conveniently providedby the phantom and side circuits of a four wire circuit such as a quad.

According to the invention a transmission system is provided which isadapted to transmit vibratory energy such as a signal or sigmils and inwhich the frequency spectrum of the signal or signals to be transmittedis scanned periodically by means of a pick up device which is adapted totraverse etfectuah ly a substantial portion of the frequency spectrum ofthe signal to be transmitted and to pick up therein portions of thesignals, said portions being or not shift-ed (e. g. by moduiation. andselection of a side band) in the frequency spectrum and then transmittedto the receiving station where said portions are restored to form anintelligible signal.

Below several arrangements embodying the invention will be describedwith reference to the accompanying drawings which illustrate the natureof the invention.

In Fig. 1 there is shown schematically an important feature underlyingthe invention.

Fi 2 and 3 are diagrams showing the frequency spectrum of the signals tobe transmitted and their position at different periods in the cycle ofoperation of the system.

Fig. a shows the general arrangement of apparatus at the transmittingend.

F 5 shows the arrangement of app-.i ratns at the receivingstation.

Fig. 6 is a modification of the arrangement of F' 4 and 5, usingoscillators of constant =3; and a pair of distributors, one at q aidingstation and one at the receiving station.

Fig. 7 illustrates in a diagrammatic form the operation of thearrangement of Fig. 6.

Fig. 8 represents an arrangement similar to the one shown in Fig. 6 andusing a plurality of pairs of distributors.

A signal such as a telephone conversation may be considered for thepresent purpose as a quantity having two dimensions, one with respect totime and the other to freqiuncy. hiorc accurately a third dimensionshould be added this being the amplitude characteristic of the signal.In some cases it may be necessary to take into account the relativephase of the various components of the signals. Therefore if a specificsignal does not exceed the frequency B and the time 7', ail the elementsof said signal Will be represented by points within the rectangle 1, 2,3, 4. of Fig. 1A.

According to the invention the frequency scanning devices select at thetransmitting station. certain portions of the rectangle 1, :2- 3, 4.Those selected portions may have any convenient shape. In the firstembodiment of the nvention hereinafter described, portions of thesignals are selected for transmissien as indicated by the hatchedportions of the rcctau l-iof 1B. According to another on :ndinnntdescribed below portions are sel cted for transmission as shown by thehatched portions of Fig. 1D.

Fig;- ED shows the shape of the selected portr of the signals in thecase in which the frequency band B is divided by two. It

should be noted however that in the embodiment shown in Fig. 6 thefrequency band is divided by 3 whereas in Fig. 8 it is divided lay-,4.:Figs. 1C and; 1E show other manners of selecting portions of therectangle 1, 2, 3,

In the. diagramofFig; 113- it is assumed that the picleup device scansthe speech range ith a uniform velocity- 'However, this is not essentialto the invention and the rate of travel ofthe pickup device may vary inClLI- ferent parts of the frequency range according to any desired lawwhich may be found to improve the quality of the signals transmitted.For instance in some cases it may be. advantageous to move the pick-upmeans with a velocityincreasing towards the upper portions" of thefrequency range B.

This result may be obtained by adapting the frequency of thescanningdevice oscillator broscillatorsof variable frequency, such forexample as those in the system of Figs. 4 and 5 described hereinafter,to vary according to a predetermined law. For instance when saidoscillators are provided with rotating condensers the shape of the vanesof said condensers may be determined so that the frequenc'ythat is tosay'the speed of scanning variesv according to the desired law. Whenoscillators of constant frequency are used as in Figs. 6 and 8 describedhereinafter, it is possible in the case of Fig. 8'to rotate the arms Rand R of the distributors at difierent speed to to provide a differentnumber of contacts on different pairs of rotating arms.

It should'be noted that blank of signals with respect to time may beprovided as shown in- Fig. 1E.

It has been assumed above that it is the scanning device which is movedwith time along the frequency band B but it is also possible to move thefrequencyba'nd B with time along the scanning means for instance bycarrying the frequency band B upon one-or several carrier frequenciesvarying according to a suitable law in function of time.

In order to obtain. effectually scanning mean movable along thefrequency band 13,.

filters of variable cut off frequencies (not shown) may be used.

It will be seen' that the-two above arrangements may be used-together,the essential being to obtain relative displacement of the scanningmeans with regard to the speech. band. i

Referring now to Figs. 2 and 3 one manner of carrying into practicethe-invention will be described.

On the axes Off and. Ofi" of Fig. 2 are represented the pass ranges oftransmitting and of receivingfilters in their relative'position in thefrequency spectrum. 1 Inthisfigure F 0 represents the frequency bandwidth of the signal to be transmitted and F3 the band of. frequencieswhich may betransmitted over the transmission medium. F 1 shows theposition of the band of frequencies to be transmitted after the firstmodulation whilst F2 represents the pass range of the transmitting andreceiving filters in their relative positions in the frequency scale.

According to an embodiment of the invention, when it is desired totransmit signals extending over a band of frequencies from f to f f f=B) over a transmission path capable of transmitting with tolerableattenuation, distortion, etc. only a band of frequeneies from f to, f (ff =b) the following system may be used,'with apparatus'such for exampleas indicated in Figs. 4 and 5 described hereinafter.

By modulation and selection of the lower side band (for example inmodulator m and filter F l in Fig. 4). the original band of frequenciesA to f is first raised to a range f to f such that f is greater, andpreferably considerably greater than (f f The purpose of this is tofacilitate elimination of the upper side band products of secondmodulation which immediately follows (for example in modulators m inFig. 4). In order that no trouble shall be experienced from the upperside band product of modulation the frequency of the first carrier mustbe removed from f by a frequency greater than (f -f and either a bandpass filter having its pass range from f to 2 must be emloyed or elsethe frequencies of the variable frequency oscillators must be chosen sothat the upper side band of the primary modulator lies outside the passband of the scanning filter described below.

The frequencies f to f will be referred to as M the first modulationband. From the nature of modulation it follows that The first modulationband M is supplied (for example from modulator m in Fig. 4) to gmodulators (for example modulators m in Fig. 4) performing the second,or variable frequency modulation. In addition to M these 9 modulatorsare each supplied With a carrier frequency from one of an 'ual number 9""of secondary oscillators (0 O and 0 in Fig. 4). The frequency. of eachoscillator is varied between the same frequency limits i and f by meansfor instance of a separate condenser: of varying value (not shown),consisting of an air condenser with one set of vanes on a commonrotating shaft making 11, revolutions per second. Both fixed and movingvanes are in the form of sectors of circles concentric with the rotatingshaft. The sectorial angle 0 of the moving vanes and the fixed vaneswith which they inter-act is the same in every case, but where'the fixedvanes on a projected View alongthe axis of the shaft are situated in thesame sector, the moving vanes of each oscillator are consecutivelydisplaced by an angle B B+b"' Arrangements may be made so that the output from each modulator is only transmitted during the time that eachcondenser is increasing in capacity (or alternatively during the timethat each condenser is diminishing in capacity). This may be achieved bythe use of commutators on the rotating shaft.

The frequency range f f of each secondary oscillator is adjusted so thatf f 7 f1 f4 fs The lower second modulation band M therefore varies fromthe position X Fig. 3 (wherein theaxis 0} represents frequencies and theaxis 0A the amplitudes) in which the frequency range is Upperfrequency=f f Lower frequency=f -f to the position Y Fig. 2 in which thefrequency range is Upper frequency=f f Lower frequency= f f Between f fand f f there is a gap of width fofe (lb 1 f4fa from Equation (2).

The lower modulation bands M from each second modulator are all suppliedto one and the same band pass filter (scanning filter),

for example filter F in Fig. 4, whose pass range extends from f f5 1' 0fs fef from Equation (3).

f8 f7 f4 fa It is therefore evident that if f1' (f4 f the upper sideband products of modulation will always be outside the pass range of thescanning filter. In the event that primary modulation may be dispensedwith.

Considering the band M from any one modulator it is evident that thescanning filter effectively scans the band n times per second, by virtueof the fact that the band M traverses the pass range of the scanningfilter every second. The speech range is therefore scanned m times persecond with q modulators. The purpose of employing a plurality ofmodulators is to ensure that the pass range of the scanning filter scansthe speech spectrum continuously, since by this means it is arrangedthat as any second modulation band M leaves the filter it is followedcontiguiusly by another second modulation The following will help tomake this clear. Let the series of second modulation bands M bedesignated as M M etc. corresponding to oscillators O 0 etc. controlledby condensers Ca, Cb, etc. When condenser Ca=0 and is just about tostart increasing, band M is at X, Fig. 3. When band M has arrived at Z,Fig. 3 condenser C has rotated through an angle since the plates of thecondensers must be so shaped that equal changes of angle give equalchanges of frequency (standard type straight-line frequency condensers).At this moment a second condenser C must start to increase and thereforelags behind condenser 0 by an angle In general there Will be a total of9 such rotating condensers each lagging behind (or leading on) the nextby 45 degrees. It is naturally desirable that the number of suchcondensers shall be finite and small. This may be achieved by arrangingthat the phase of the (0+1) th. condenser is 360 behind the firstcondenser 0,. Since the condensers are formed by the capacities betweenmoving and fixed plates in the shape of sectors of circles concentricwith the axis of the shaft carrying the moving plates, the smallestintegral value of g is given by the equation B B+b where 0 is notgreater than 180.

Example 1 b /;B. Therefore,

Putting 9 2, 0=270 which is not allowable.

Putting q=3, 6= 180. 120.

Ewample 2 1 o 6 B. .g %0 360 Putting 9 3, 0 160", 4 =120.

If g is made equal to unity and 6=180 then incomplete scanning occurs,since each second modulation band leaves the filter pass rangecompletely before another enters it.

Such a system may give tolerably good results.

The output from the band pass filter (F in Fig. 4) which lies in therange f to f is modulated (for example in modulator m in Fig. 4) with asteady frequency of the value of f -f (for example from oscillator 0 inFig. 4) and so reduces the band frequencies to the range f to f. (whichis the past range of filter F3 in Fig. 4), for transmission to thereceiving end of the system (over a line or medium L in Figs. and 5) Atthe receiving end the iifrcqnencics received are beaten (for example indemodulator D in Fig. 5) with a steady frequency of the value f f (fromoscillator G in Fig.

and then traverse a band pass filter having the range f to f The outputfrom the hand is supplied with time selectivity to q demodulators, (forexample demodulators D in F ig. 5). each demodulator being supplied withan oscillator (G G and G in Fig. 5) whose frequency at any instant isthe same as that of the corresponding modulating oscillator at thetransmitting end.

Each modulator is connected in circuit only during' the same time thatits corre sponding modulator (0 O and 0 respectively, in Fig. 4) isoperative at the transmitting end. For the purposes of the above it isassumed that times at the receiving end are the same as those at thesending end when displaced by an amount corresponding to the mean delayof the circuit.

The combined output from the demodulators (D in Fig. 5) lies in therange f to f and it is necessary to employ a third oscillator anddemodulator (G and D in Fig. 5) to reduce this to the range 7"; to f(which is the past range of filter F0 in Fig. 5).

Synchronism between the events at the transmitting and receiving endsmay be accomplished by any known means such as the transmission of 50cycle current along the transmission path.

The resulting product at the receiving end is effectively that whichwould he produced by causing the pass range of a variable band filter ofconstant band width to traverse the original band Zn times per second.In this case each part of the range would be trans" mitted during onlyof any time considered. It is therefore proposed to substitute themissing parts, by re peating these parts which are reproduced anyrequired number of times. If

then repetition must be made once; if

two repetitions must be made and so on. Repetition may be secured bysupplying the third demodulation products (from filter F O in 5) to adelay network (N in Fig. 5) and combining (for example in amplifiers Aand A in Fig. the voltages taken from different points along its length,for transmission to the receiving circuit, indicated at R in Fig. 5.

then it is only necessary to combine the voltages from the input andoutput of a network having a delay of 2q'n seconds.

then it is necessary to combine the input voltage with voltages delayedby 1 35?, and

3 an seconds.

B 1; 4 then the delays are 1 2 m e and 3 m and so on.

The general arrangement of apparatus at the transmitting end is shown inFig. 4, as indicated above.

O and m are respectively the primary oscillator and modulator the speechband to be compressed being supplied at S.

F1 is a band filter passing the range f to 7'2- O 0 and 0 the secondaryoscillators and m are the secondary modulators.

F2 is a band filter passing the range from f to O and 772 arerespectively the tertiary oscillators and modulators.

F3 is a band pass filter passing the range from i to f The compressedband is supplied to the line at L.

The general arrangement at the receiving end is shown in Fi 5, asindicated above.

G and D are respectively the oscillator and modulator converting theband from f to f to the range f, to f and F2 is a band filter passingthis range.

G G and G are the variable frequency oscillators corresponding to O Oand and D are the demodulators cor responding to m G is synchronizedwith 0 G with O and G with 0 so that each to each the frequencies arethe same at instants separated in time by the time delay of thetransmission path. The synchronizing means are not shown.

F1 is a band filter passing the range from f to 72.

G and D are respectively an oscillator and modulator converting the bandfrom f, to f to the range f to f F0 is a band filter having its passrange from f, to f N is a delay network for producing repetitions.

A A and A are one-way devices, which may conveniently be amplifiers ofhigh impedance input with their outputs parallel.

The restored band is delivered at R.

Three oscillators have been found a convenient number but it should beemphasized that the invention is not limited to this particular numberof oscillators. It will be seen from the above that an important featureof the above system is the use of multiple modulation which in theparticular instance described is triple modulation and demodulation. Inorder to show another way of carrying out the invention, in thefollowing there will be described a system in which the frequency bandof the signals is moved step by step in front of a plurality of scanningdevices provided with frequency changing means and said signals orportions thereof are transmitted to the receiving station either in turnover one channel with synchronizing means at the sending and receivingstations or over a plurality of channels without synchronizing means atthe terminal stations.

The velocity of scanning must be large enough so as to preserve theintelligibility of the signals. For instance when it is desired totransmitspeech it has been ascertained that each individual frequency ofa composite wave representing a vowel or consonant lasts for aconsiderable time greater than one hundredth of a second withpractically constant amplitude.

It should be noted that in systems of the type herein set forth it maybe necessary to provide means for compensating for phase distortion orphase compensation. In some cases it may be necessary to reducetransient effect in the systems.

Considering more closely the question of phase distortion, let N be thenumber of times the speech band is scanned per second, then =q where gtotal number of IIlOflllhltOrs n number of revolutions per second of thevarlable condenser spindle.

Hence the time for one scanning is Let B=band width of original speechthen the velocity of scanning is BN cycles per second. If new anyfrequency is displaced in time from its original position by S seconds,on demodulation the frequency will be displaced by F cycles per secondwhere (6) F=B-N-S cycles per second for example considering a practicalcase and taking cycles as the maximum permissible displacement of thereceived speech 1 20 and then If we suppose a speech band width B equalto 2000 cycles and 3 modulators (9 3) we get 1 8 soon (8) The valuegiven by (8) represents:

(a) The maximum permissible deviation is time delay between any twofrequencies in the range considered and also,

(6) The maximum permissible total time delay at any one frequency.

Two methods of adjustment may be used for correcting for an excessivetime delay:

(1) By the use of two sets of oscillators vanes moving in correctrelative phase. For illustrating the above point suppose that the timedelay for which it is required to adjust is S microseconds, then theleading edge of one set of receiving condenser vanes must lag behind theleading edge of one set of transmitting condenser vanes by a time equalto S microseconds that is it must lag by Sn 360 degrees.

If the condenser vanes of the receiving apparatus lag behind thetransmitting ones by any integral multiple of 120 (taking q=3 as in theabove example) it simply means the two condensers are occupying similarpositions and that the same capacity will be inserted in the oscillatorscircuit at both ends altl'iough a different set of "anes may be inposition. From the considerations above it will be seen that multiplyingthe speed by 2 will have the same effect as increasing the lag of thereceiving vanes by 120 and consequently if the circuit has beencorrectly adjusted for any given scanning speed then this speed may bemultiplied by any integer Without upsetting the adjustment.

(2) By the addition of a uniform time delay to the value of S. It hasbeen shown above that if both receiving and transmitting vanes canoccupy the same position in space, that is no adjustment is necessary.Consequently, if a uniform time delay is added to the value of S to makethe total delay equal to complete adjustment for the particularfrequency under consideration will be obtained. Such a uniform timedelay may be obtained by the use of lattice networks.

The deviation in time delay may be phase compensated for instance bymeans of lattice networks to a degree of accuracy determined by thescanning means used.

In the systems hereinafter described in connection with Figs. 6, 7 and8/, it will be assumed that it is desired to divide the frequency bandto be transmitted by three, but it will be understood that the frequencyrange may be divided to any desired extent.

A schematic of this arrangement is shown in Fig. 6. The signals, forinstance speech, extending over the range 0-2700 cycles, are impressedon the input I of the system and passed to a distributor comprising abrush R1 rotating at say 100 revolutions per secend which makes contactwith each of segments 8,, S S for one third of each revolution.

It will be seen that the speech energy is applied to band filters BF BFBF in turn the said filters, filtering respectively the ranges 200-900,900-1600, 1600-2700 cycles.

Thus during the first 300th of a second energy in the range 200-900 isapplied to device AH which may be an amplifier; during the second 300thenergy in the range 9-00-1600 is applied to modulating or frequencychanging means AH and during the third 300th energy in the range1600-2700 is applied to modulating or frequency changing means AHModulating means AH and AH may for instance include amplifiers, eachcoupled with a heterodyne oscillator. The frequency of the oscillatorincluded in the modulating means AH is 700 cycles, so that the resultantbeat frequencies produced in AH lie in the range 200-900. The oscillatorin the modulating means AH generates 1400 cycles, which produces beatfrequencies between 200 and 900 cycles also. AH requires no oscillator,since its energy already lies in the range 200-900. The outputs of AH AHand AH are then transmitted down the transmission channel TC.

On Fig. 6 AN and VN are networks designed to equalize the attenuationand Velocity of transmission of the transmitting channel for the range200-900 cycles, A is an amplifier.

At the receiving end, the line is connected through an amplifier A to abrush R synchronized with R The three segments associated with R areconnected to an amplifier AH and demodulators AI-I and AH,. Since R andR are synchronized, all energy received from AH is passed into AH allenergy from AH goes to AH and so on. Demodulators AH and AH includeamplifiers, each coupled to a local oscillator which restores the speechenergy to its correct frequency range. AH, does not require anoscillator, since its speech energy is already in the correct range.

The outputs of AH,, AIL, AH are now exact copies of the inputs to AH AHAH which will not produce intelligible speech. For instance, during onethree hundredth of a second, energy in the range 200-900 is receivedfrom AH but during the next two hundredths no energy in this range isreceived.

In order to compensate for the missing portions of the speech band, forexample the portion 200-900 cycles, this portion is reproduced duringthe next two intervals during the time brush R passes from contacts S Sto contact S thereby greatly improving the intelligibility of thespeech, since as explained above, each frequency lasts for a long timewith practically constant amplitude.

This may be achieved by passing the output of AH through a delay networkDN, having a total delay of about one hundredths of a second. A coil Cis coupled to the first coil of the network, and a second coil C to acoil in the network having a delay of one three hundredth of a secondbehind the first coil, and a third coil C coupled to a coil in thenetwork with a delay of two three hundredths behind the first coil.These three coils, C C C are placed in series, and connected to theinput of an amplifier, A of very high input impedance, so that thecurrent in C C C is negligibly small.

During the first 300th of a second C, picks up frequencies received bythe network from A during the second 300th, C, no longer does so, butthe Wave train has then reached G which picks up during this interval,during the third interval C picks up in a similar Way. At the beginningof the fourth interval, a fresh wave train is received from AH andpicked up by C The voltage impressed on A is therefore continuous, andowing to the long persistence of each frequency in speech, practicallyindistinguishable from the original speech wave.

Frequencies in the bands 900-1600, and 1600-2700 received from AH andAH, are repeated in a similar way by the delay network.

The diagram in Fig. 7 shows the wave trains received from AH AI-I AH bythe delay network. In the figure the axis 0 t represents time and theaxis 0 A the amplitude. The time T is the period of the operation andthe waves WS S WS are the waves sent over the contacts S S2, S of thedistributor.

In order to avoid transient effects in the filters BF BF etc. it isdesirable that the input to these filters should be connected to animpedance equal to the characteristic impedance of the filter itself,and that it should be closed through this impedance regardless of theposition of R This is brought about by inserting a small amplifier (notshown) before each filter.

It is essential that the time taken for each speech band to travel fromR to the line should be substantially the same. As the delays introducedby the filters may vary with frequency, it may be necessary to equalizethe velocity of transmission by phase compensating networks introducedbetween each filter and its associated amplifier.

In the following description, the term contact time is used to indicatethe time during which the brush R is on one contact.

The contact time must be short compared to the shortest transient it isdesired to transmit; it must also be large compared to the time of onecomplete cycle at the lowest frequency to be transmitted if distortionis to be avoided. Distortion at the change over at the end of eachcontact time will always occur unless the transmitted frequency is anexact multiple of the interruption frequency. But provided the former ismuch greater than the latter, this will only produce a superimposed humof interruption frequency, which can easily be removed for instance by afilter. There will also be harmonics of the interruption frequency whichwill, however, be small and the lower harmonics which are below thespeech band can be cut out by the same filter which removes thefundamental. Those which occur within the speech band are generally toosmall to cause trouble.

From the above, it appears that in an arrangement for dividing thespeech band by n, the contact time must not be greater than seconds.This is based on the assumption that the shortest transient lasts formore than 1/ 100th second. In addition, the contact time must be longerthan the time of about four or five cycles at the lowest frequency it isdesired to transmit.

These two conditions are irreconcilable, if the speech band is 300-2300cycles and n is 3 or 4. To overcome this difficulty, a modification ofthe above arrangement can be used. This is shown in Fig. 8. The speechrange is first divided into two bands, 3004300, 13o0 2300, by twofilters BF, and each band is applied to a rotating contact R or R withfour contact segments. The output from each of these is treated in thesame way as before, and the two outputs are transmitted on two adjacentfrequency channels, say 200-450, and 450-700 cycles per second.

The speed of R is about 25 revolutions per second, so that its contacttime is l/ 100th second. Thus even at 800 cycles, three complete cycleswill occur in one contact time. R rotates at 100 revolutions per second,its contact time being 1/400th second. It'thus includes more than threecycles for a speech frequency of 1.300 cycles per second.

Viththe above arrangement, it may be desir-able to keep the two channelsseparate until after the delay network stage. It will be necessary toprovide a delay networkfor each channel, but the design of thesenetworks will be simpler because the frequency band dealt with by eachis narrower.

The band which passes through the delay network may be still furtherreduced by inserting the network before the received energy is separatedinto the different speech bands. In this case only one network dealingwith the transmitted frequency range 300-700 cycles will be required.

The brushes at the sending end may be driven by synchronous motors (notshown), and the AC supply driving them is transmitted down the line. Atthe receiving end it is amplified and used to drive the rotating brushesR and R Exact synchronism is thus obtained. lVhere two brushes are usedat each end, two synchronizing frequencies must be transmitted. Thesewill be at a very low frequency sufficiently removed from the speechband to prevent interference between the two.

In the figures a certain disposition of the various pieces of apparatusis shown which convenient in certain cases: it will be understood,however, that the sequence of the various operations may be modified inany manner which may be found convenient. For instance the distributoror distributors may be inserted after the frequency selecting and/orfrequency changing devices. Likewise the delay network or networks maybe inserted at the receiving stations in any convenient position withregard to the other terminal apparatus. Other modifications will beobvious to those skilled in the art. Although mechanical distributors ofthe rotary type are shown in the above embodiments, We may use insteadany other electrical or mechanical switching means well known in theart, such as arrangements of thermionic valves, etc.

What is claimed is:

1. The method of transmitting signals with reduced width of thefrequency band transmitted, which comprises periodically scanning asubstantial portion less than the wnole of the frequency spectrum of thesignal to be transmitted, and transmitting only the frequency componentsderived from said portion.

2. The method of transmitting signals which comprises periodicallyscanning a substantial portion of the frequency spectrum of the signalto be transmitted and picking up a fraction only of the signalcomponents in said portion at a time, and altering the frequency of atleast a portion of the components picked up.

3. A signal transmitting system comprising scanning means forperiodically traversing effectually a substantial portion of thefrequency spectrum of the signal to be transmitted, to pick up portionsof said signal, shifting the frequency components so picked up to occupya narrower total band and transmitting said band of components.

4. A signal transmitting system comprising scanning means forperiodically traversing effectually a substantial portion of thefrequency spectrum of a signal to be trans mitted and picking upportions of said signal, and means for shifting at least some of saidportions in the frequency spectrum.

5. A signal transmitting system comprising scanning means forperiodically traversing effectually a substantial portion of thefrequency spectrum of the signal to be transmitted and picking upportions of the signal, means for transmitting the latter portions to adistance, and means for reprodueing a number of times at least some ofthe transmitted portions to compensate for the portions of the signalswhich are not transmitted.

6. A signal wave transmission system comprising scanning means movableperiodically with time relatively to the frequency spectrum of thesignal to be transmitted for picking up such portions of the frequencyspectrum of the signal to be transmitted that the number of differentfrequencies as well as frequency band width necessary for transmittingthe signal is reduced, a transmission channel, means for transmittingsaid portions over said channel in turn with time selectivity, andsignal receiving means for producing from the transmitted waves a signalrepresentative of the signal to be transmitfed.

7. A signal transmitting system comprising a band pass filter with fixedcut-off frequency, a source of carrier frequency waves,

means for modulating the signal to be transmitted with said carrierfrequency waves, means for transmitting the modulated Waves to saidfilter, and means for causing such periodic variation of the frequencyof said carrier Waves that the essential frequencies of the signal to betransmitted during a band of the varying carrier frequency are broughtWithin the pass range of said filter and the lrequency spectrum of thesignal is scanned.

8. A system for transmitting signals comprising a plurality ofmodulators, means for supplying said signals to said modulators, meansfor supplying to said modulators carrier waves so varying that during acycle of the variation portions of the modulation products fall within arange of frequencies of width smaller than the range of frequenciesoccupied by the corresponding portions of the signals to be transmitted,and means for transmitting said portions of the modulation product to adistance.

9. A system for transmitting signals comprising a plurality ofmodulators, means for supplying said signals to said modulators, meanssupplying to said modulators carrier waves so varying that during acycle of the variation portions of the modulation prod ucts fall withina range of frequencies of width smaller than the range of frequenciesoccupied by the corresponding portions of the signals to be transmitted,and means for shifting said portions of the modulation products in thefrequency spectrum.

10. A signal transmission system comprising a transmitting station, areceiving station, a plurality of modulators at said receiving station,means for impressing the signals to be transmitted on said modulators inturn and at a suitable rate, means for supplying to said modulators,respectively, modulating waves having constant frequencies of suchvalues that the signals impressed on the modulators are brought within afrequency band width smaller than that of the original signals, meansfor transmitting the signals thus reduced in their frequency band widthto said receiving station, a plurality of modulators at said receivingstation, means for synchronously impressing the signals transmitted to'said receiving station on the latter modulators, and means for supplyingthe latter modulators with waves of suitable frequencies for restoringthe transmitted signals to their original frequency range.

11. A signal transmission system as set forth in claim 10. the means atthe trans mittiug and receiving stations for impressing the signals onthe modulators comprising a pair of distributors, one at each station,adapted to rotate in synchronism, and the means at the transmittingstation for impressing the gnals on the modulators comprising also aplurality of band pass filters selecting portions of the signals to betransmitted.

12. A system as set forth in claim 10, comprising a delay network atsaid receiving station for repeating a number of times the signalsarriving at the receiving station, to compensate for the portions of thesignals which are not transmitted. 4

13. A system as set forth in claim 10, the means at the transmitting andreceiving stations for impressing the signals on the modulatorscomprising a plurality of distributors at each stat-ionadapted to rotatein synchronism, a number of pairs of the distributors being used fortransmitting the higher frequencies of the signals to be transmitted andthe remaining pairs of distributors being used for transmitting thelower frequencies of the signals.

14. A signal transmitting system comprising scanning means forperiodically and at varying velocity traversing effectually asubstantial portion of the frequency spectrum of the signal to betransmitted, to pick up portions of said signal, shifting the frequencycomponents so picked up to occupy a narrower total band and transmittingsaid band of components.

15. The method of conserving the frequency band width employed intransmitting speech which comprises selecting components representing(liiferont portions of the frequency spectrum of speech from differenttime elements of the speech, shifting the frequency of certain of thecomponents so selected and transmitting them to the exclusion of othercomponents.

16. The method which comprises selecting signal components representingdifferent portions of the frequency spectrum of a signal from differenttime elements of the signal to the exclusion of other components andrelatively changing the frequency of said selected components to causethe signal com ponents to occupy a narrower total band of frequenciesWhile preserving their original time relation.

In witness whereof, We hereunto subscribe our names on the 24th day ofOctober, 1929, and the 15th day of November, 1929, respectively.

EDWVARD KENNETH SANDEMAN. ROBERT OWEN CARTER.

